What's it called and ............

In these parts it's referred to as aluminum tape but alot of people just say foil tape.

the 3M website actually has technical data on their aluminum foil tape products and there are a few types for applications. I have a roll that says it's good for 300 F but it also says best applied 24 months after purchase so if buying on EBAY make sure it's not old stuff where the adhesive has dried out.

Here is some information on what to look for in that foil tape....

Are you looking to seal ducts or seal some sort of roof? even though its still a metal to metal seal, two completly differnt products.

A picture would help.

Inside or outside? I am Leary of the life of the stickiness, but the stainless steel foil tape I put on a car held up fairly well.

Greg

Foil tape won't last. Like duct tape, the adhesive decays.

For your application, consider slathering some roof repair muck over the cracks. Water, heat, cold, and vermin resistant.

A Company called Eternabond makes a variety of foil tapes for roofs. I've never seen their products before and have of course never used them but maybe they are worth investigating on google just to see.

Steve, I can't figure out what it is you are doing, but it doesn't sound like conventional duct work. Think you may want to look at a product called duct sealant, sometimes called hard cast. It is applied with a paint brush, putty knife, roller, or caulk gun and can be reinforced with nylon mesh similar to what Dryvit uses with the base coat, called scrim. This stuff will work fine outdoors or indoors, most are designed to work from below freezing to about 200. We use Childers CP 11. This will be much easier to work with than the tape and there will be much less chance of not sticking, etc. The latex mastics with a throw away chip brush are great.

Here is an example of what I suggest: